An avalanche photodiode utilized in optical communication is a semiconductor light-emitting element whose light sensitivity is raised by providing a layer for avalanche-multiplying carriers obtained through photoelectric conversion, in addition to a light absorption layer for performing the photoelectric conversion; it is required that the dark current of an avalanche photodiode is low and the reliability thereof is high.
Most of avalanche photodiodes are each formed of a compound semiconductor and depending on the structures, roughly divided into a mesa-structure and a planar-structure avalanche photodiode. In a mesa-structure avalanche photodiode, a mesa is formed on a substrate, and a structure including a p-n junction is provided in the mesa; a breakdown is liable to occur on the surface around the mesa. In general, in order to suppress the breakdown, an inclined structure is employed; moreover, by, for example, employing a structure in which an embedded layer, which is supposed to become a high-resistance portion, is provided around the mesa, a dark current is designed to be suppressed low (e.g., refer to Patent Document 1)
In a planar-structure avalanche photodiode, by providing a selective diffusion region, a p-n junction is formed; an edge breakdown in the edge portion of the p-n junction poses a problem. When a current flows in the edge portion, the reverse voltage across the p-n junction in the light-sensitive unit located in the center of the avalanche photodiode hardly increases, even though the voltage is increased; therefore, the function of an avalanche photodiode cannot be exerted. Accordingly, measures have been taken in which, for example, a high-resistance guard ring is provided in the edge portion through impurity implantation or the like (e.g., refer to Patent Document 2).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2002-324911 (FIG. 1)
[Patent Document 2] Japanese Patent Application Laid-Open No. 1995-312442 (Pages. 4 to 6, FIGS. 4 and 6)